Added defensive code to DNSResolver to avoid ifaces down after crash

Adding defensive code as a workaround to DNSResolver issue. The inner problem is still there (only one iface with DHCP), but this will avoid the crash, so ifaces won't be down after saving Network changes.

I've tested this workaround several times in 3.2, and it worked fine for me, avoiding the crash, but adding always a new redis entry.